The project is a continuing series of investigations of the crystal structures of metal-peptide complexes. The aim of the work is to explore the geometry of metal-peptide binding in order to set up models for metal-protein interactions in more complicated systems (such as metal-activated enzymes, 'biuret' reactions of proteins, and heavy-atom derivatives prepared for protein crystal structure analyses.) Reviews of these and related researches since 1959 have been published in Adv. Protein Chem., 1967,22,257 and in 'Inorganic Biochemistry'. The compounds whose X-ray crystal structure analyses were carried out during the first few years of the project included Cu(II) complexes of glycine peptides from glycine through tetraglycylglycine; Cu(II) complexes of dipeptides with a variety of side-chains; Ni(II) complexes of glycine peptides from glycine through triglycylglycine; cationic and anionic complexes of Co(III) with glycylglycine; and some complexes of Cu(II), Ni(II) and Co(III) with mixed amino-acid, peptide, imidazole and triethylenetetramine ligands. The work is being extended to complexes of Pt, Hg, Ag, Cd, actinides and other 'heavy' atoms. Series on which substantial progress has been made are the Pt(II) complexes of glycine, methionine, glycylmethionine and histidine; the Ag(I) complexes of glycine, methionine, imidazole and glycylglycine; the Cd(II) complexes of glutamic acid, methionine, asparagine, penicillamine, imidazole, glycylglycine and glycylglutamic acid; and a number of Tl(I) complexes of cysteine. In several cases, the structure analyses have been carried out by a combination of X-ray and neutron diffraction.